1. Field of the Invention
The present invention relates to a glass substrate for a magnetic disk and a magnetic disk apparatus.
2. Description of the Related Art
In responding to the recent trend for magnetic disk apparatuses, such as hard disk drives, of fast-paced improvement of information recording density, a fly height of a magnetic head or a magnetic head slider that opposes the surface of a magnetic disk has been reduced. A load/unload method, see for example, Japanese Patent Application Laid-open No. 2001-319326, is being adopted to prevent adhesion of the magnetic head to the magnetic disk due to reduction of the fly height. Various technologies are available to prevent head crash, which consequently damages the magnetic disk, in the load/unload method, that is, to prevent the magnetic head slider loaded onto the magnetic disk from a ramp from hitting the surface of the recoding zone. For example, Japanese Patent Application Laid-open No. 2001-319326 discloses a technology for preventing a direct contact between the magnetic head slider and the recording-zone surface by forming a convex portion as an initial floating area in a loading area that locates away from an outer circumference of the recording zone of a magnetic disk.
In a polish-finished glass substrate for a magnetic disk, a protrusion called a ski jump appears on a peripheral portion of a main surface. According to Japanese Patent No. 3184261, if the maximum height of a ski jump less than 0.35 micrometer, a magnetic head slider floats properly over the ski jump, which allows expansion of a recording zone on the magnetic disk.
Japanese Patent Application Laid-open No. 2003-242627 discloses expanding a recording zone by improving the flatness of a slope of a ski jump. Specifically, when the shape of the slope of the ski jump is defined as a distance called a radial curvature (RC), the RC of equal to or less than 50 nanometers allows an improvement of the flatness of the slope of the ski jump, thus expanding the recording zone.
However, even if a ski jump is formed in accordance with the conventional technique, a magnetic head slider still has a chance of coming into contact with a main surface of a magnetic disk when the magnetic head slider is loaded or unloaded, so that the magnetic head slider can not stably float over the magnetic disk, leading to head crash.